Graças às dicas do @George, finalmente consegui instalar o obturador . Desativei todos os repositórios na lista que mencionam precisas . Surpreendentemente, cada repositório mencionou, substituiu-o pela seguinte lista
###### Ubuntu Main Repos
deb http://us.archive.ubuntu.com/ubuntu/ xenial main restricted universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ xenial main restricted universe multiverse
###### Ubuntu Update Repos
deb http://us.archive.ubuntu.com/ubuntu/ xenial-security main restricted universe multiverse
deb http://us.archive.ubuntu.com/ubuntu/ xenial-updates main restricted universe multiverse
deb http://us.archive.ubuntu.com/ubuntu/ xenial-proposed main restricted universe multiverse
deb http://us.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ xenial-security main restricted universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ xenial-updates main restricted universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ xenial-proposed main restricted universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse
Em seguida, tente novamente o sudo apt-get update
e, em seguida, sudo apt-get install shutter
para instalá-lo com êxito.